1. Field of the Invention
The present invention relates to a phase shift device and a laser apparatus utilizing the same, and more particularly to a phase shift device for dividing a passing area of a laser beam into plural areas and giving a predetermined phase difference between the laser beams passing through the areas, thereby regulating the beam spot diameter of the laser beam for the purpose of image formation of a high precision or correcting astigmatism of the laser beam, and a laser apparatus employing such phase shift device.
2. Related Background Art
In recent laser apparatus for input/output of image information utilizing the laser beam, there is desired capable for forming or reproducing image information with a high resolving power.
A high resolving power can be achieved, in the formation or reproduction of image information, by reducing the beam spot diameter of the laser light (laser beam) at the image input or output, and it is already known that a reduction in the beam spot diameter can be achieved by the use of an optical system with a large numerical aperture (NA).
FIG. 1 is a schematic view of a conventional optical scanning apparatus employed, for example, in a laser beam printer.
Referring to FIG. 1, a laser beam emitted from a light source unit 91 consisting, for example, of a laser is converted into a substantially parallel beam by a collimating lens 92, and enters a rotary polygon mirror 93. The mirror 93 is rotated in a direction indicated by an arrow, with a constant high speed, whereby the laser beam, entering a point P on a reflection face 93a of the rotary polygon mirror 93 is reflected and put into a scanning motion on a main scanning plane, thus entering an f-.theta. lens 94 constituting a focusing optical system. Emerging from the lens 94, the laser beam is focused onto a scanned plane 95 and scans the plane linearly with a substantially constant speed.
For example, in an image formation on the scanned plane composed for example of a photosensitive member, a higher resolving power can be attained by increasing the numerical aperture of the f-.theta. lens system 94 thereby reducing the diameter of the beam spot 96 focused on the scanned plane 95.
However an optical system with a large NA is very difficult to construct, and NA=1 cannot be exceeded in theory by an optical system in air. Also an increase in the NA drastically reduces the depth of focus, whereby the positional tolerance of image plane becomes narrower, thus rendering the manufacture and adjustment of the apparatus extremely difficult.
In general, the beam spot diameter and depth of focus on the image plane in a laser apparatus employing a laser light source are given by the following equations: EQU Spot diameter=k.lambda./(2.multidot.NA) (a) EQU Depth of focus=.+-..lambda./(2.multidot.NA.sup.2) (b)
wherein .lambda. is wavelength, k is a constant (k.gtoreq.1.64) indicating the level of beam intensity in the pupil periphery of the imaging optical system, and the beam spot diameter is 1/e.sup.2 of the peak intensity.
As an example, Tab. 1 shows the beam spot diameter and depth of focus on the scanned plane, for different values of NA calculated with .lambda.=0.78 .mu.m and k=1.7 in the equations (a) and (b).
TABLE 1 ______________________________________ NA of Spot Depth of optical diameter focus system (.mu.m) (.mu.m) Remark ______________________________________ 0.01 66.36 .+-.3900 Laser beam printer 0.02 33.15 .+-.975 0.05 13.26 .+-.156 0.1 6.63 .+-.39 0.5 1.33 .+-.1.56 Objective lens for optical disk 0.95 0.70 .+-.0.43 Objective lens for microscope 1.0 0.66 .+-.0.39 Limit value ______________________________________
Also the semiconductor laser employed as the light source in the laser apparatus is associated with astigmatism because of the structure thereof. More specifically, as shown in FIG. 2A, the size of the light emission area is different in a direction parallel to the junction plane of semiconductor device (x-direction) and in a direction perpendicular thereto (y-direction). Consequently the radius of curvature of the wave front at a distant point from the laser light source is different in the parallel direction and the perpendicular direction, as shown in FIG. 2B.
For this reason, when the light beam from the laser light source is focused by an axis-symmetrical lens system, there result different best focus positions Px', Py' respectively in the x- and y-directions, thus giving rise to a so-called astigmatism .DELTA. or .DELTA.'.
Conventionally the astigmatism is corrected by positioning a cylindrical lens in the optical path and adjusting the position thereof.
For example in the laser apparatus for use in a laser beam printer, as shown in Tab. 1, the depth of focus becomes smaller than .+-.1 mm if the numerical aperture of the imaging optical system is 0.02 or larger, namely if the spot diameter becomes 33 .mu.m or smaller.
In general, with a depth of focus equal to or smaller than .+-.1 mm, the manufacture of apparatus becomes extremely difficult as the positional setting or planarity of the photosensitive member becomes critical. Consequently it is generally difficult to select a numerical aperture in excess of 0.02 for the imaging optical system.
Also in the objective lens for optical disk or for microscope, a beam spot diameter of about 0.7 .mu.m is generally considered as a limit, as a smaller diameter is theoretically difficult to achieve.
As explained in the foregoing, it has been very difficult theoretically and mechanically, to obtain a laser beam of a small spot diameter in the conventional laser apparatus.
Also the correction of astigmatism, resulting when an axis-symmetrical lens system is combined with a semiconductor laser, with a cylindrical lens requires a large curvature, so that the manufacture of the cylindrical lens becomes difficult.